In control arrangements of the type to be described are used to regulate the pressure of the air spring bellows in pneumatically cushioned commercial vehicles so that they are capable of withstanding heavy loads. Normally, the vehicles have three axles for carrying the heavy load with two of the axles fastened together into a double axle set. When vehicles of this type are in general use, all of the axles do not always sustain the maximum load, i. e., the vehicle periodically runs empty or is very lightly loaded so that one axle could easily handle the load.
As shown and described in the published German patent application No. DE 29 25 190, the second axle of the double axle set is periodically relieved of stress to minimize wear. This is accomplished by establishing the pressure in the air spring bellows of the second axle in accordance with the pressure in the air spring bellows of the first axle.
In the construction as disclosed in the above German patent application No. DE 29 25 190, a pressure regulating valve is adapted to have a movable control plunger. The plunger is constructed as a step-piston which moves against the force of a governing spring. The steps of the piston having effective surfaces which are in equal size and are acted upon in the same direction by the pressurized air from the air spring bellows of a vehicle axle. The first control piston regulates a first control chamber which is connected to the two air spring bellows of the first vehicle axle by a changeover valve. The changeover valve is constructed in such a manner that the pressurized air from the two air spring bellows reaches the first control chamber when the air spring bellows and connecting lines are intact. If an air spring bellow is totally or partially inoperative, the function of the pressure regulating valve is entirely maintained by the pressurized air from the second air spring bellow. The first control chamber can be connected to a second control chamber by a multi-way valve which is connected to the air spring bellows of a second vehicle axle. The second control chamber is regulated by a movable control piston which is set against the force of a spring and is acted upon by the pressurized air from the air-spring bellows of the second vehicle axle and which is in opposition to the first control piston. The first control piston and the second control piston are connected to each other by a drag link which is part of the multi-way valve. The special construction and connection between the two control pistons in the pressure regulating valve results in a specific operating performance of the pressure regulating valve possible.
However, such a previous arrangement has the disadvantage of being relatively expensive to construct. Although it is certain that when the pressure in one air spring bellow of the first vehicle axle fails,. the pressure in the air spring bellows of the second vehicle axle will continue to be regulated without interruption, however, if both of the air spring bellows of the first vehicle axle fail, the air spring bellows of the second vehicle axle will also lose pressure.